1. Field of the Invention
The present invention relates to a method and an apparatus for controlling ink ejection of an ink jet printer.
2. Related Art
As an actuator for actuating a printer head mounted on an ink jet printer, a piezoelectric element, which expands/contracts in response to application of a voltage to change the volume of an ink reservoir formed within the actuator is used so that ink within the ink reservoir is pressurized and ejected from the actuator to the outside.
This type of actuator denoted by reference numeral 40 in FIG. 8 comprises a base 41, a piezoelectric member 42, a diaphragm 43, a cavity plate 44 and a nozzle plate 45.
The nozzle plate 45 is a flat plate having a number of (e.g., 128) ink ejection holes 45a defined therein in two lines D and E. The cavity plate 44 includes two sets of L-shaped ink ducts 44a, and ink reservoirs 44b diverged at a right angle from the ink ducts 44a. The number of reservoirs 44b corresponds to the number of ink ejection holes 45a in nozzel plate 45. Each of the ink reservoirs 44b is communicated with one of the corresponding ink ejection holes 45a.
The piezoelectric member 42 includes a number of (e.g., 128) piezoelectric elements 42a so as to expand/contract the respective ink reservoirs 44b. The diaphragm 43 is flexible and isolates the piezoelectric member 42 and the cavity plate 44 from each other.
The base 41 supports the above parts of the actuator 40. Two outward paths 41a and two inward paths 41b are formed through the base 41, the piezoelectric member 42 and the diaphragm 43 in order to circulate ink through an ink tank (not shown) and the ink ducts 44a.
As shown in FIG. 9A, each ink reservoir 44b formed on the cavity plate 44 is coupled to the ink duct 44a through a communicating path 44c, and has on its lower portion an orifice 44d formed to be in communication with the ink ejection hole 45a.
With an application of a drive voltage, the piezoelectric element 42a is expanded in direction shown by arrow X to thereby contract the volume of the ink reservoir 44b as shown by a dashed line Y. When the drive voltage thus applied is released from the piezoelectric element 42a, the piezoelectric element 42a is contracted and returned to the initial state.
In this actuator 40, ink is supplied with pressure from an ink tank (not shown) through the pair of outward paths 41a to the pair of ink ducts 44a to fill the ink ducts 44a with the ink. Then, when the drive voltage applied to the piezoelectric element 42a disappears, the piezoelectric element 42a contracts to introduce the ink from the ink duct 44a through the communicating path 44c to the ink reservoir 44b, thereby filling the ink reservoir 44b with ink. When the drive voltage is applied to the piezoelectric element 42a to contract the volume of the ink reservoir 44b, whereby ink is ejected through the orifice 44d to the outside.
While the printer head (not shown) scans a print medium (not shown), ink is ejected from each ink ejection hole 45a by expanding/contracting each piezoelectric element 42a based on desired print data, whereby a desired image can be printed on the print medium.
As the ink is thus ejected from the actuator 40 by expanding/contracting each piezoelectric element 42a, a crosstalk phenomenon in which an ink ejection amount is decreased at the same time the ink ejection speed is decreased may frequently occur if a large number of piezoelectric elements 42a expand/contract simultaneously. The crosstalk phenomenon is classified into two types. That is, the crosstalk phenomenon is caused by the insufficient rigidity of the cavity plate 44 or by change in pressure wave within the ink.
More specifically, in one type of crosstalk, when a large number of piezoelectric elements 42a are expanded/contracted, e.g., all piezoelectric elements 42a contract to fill the ink reservoirs 44b with ink, as shown by arrows O in FIG. 9B, the ink flows from the ink duct 44a into the ink reservoirs 44b. When all piezoelectric elements 42a expand to apply pressure to the ink to eject the ink from the ink ejection holes 45a, much pressure of the ink is simultaneously applied to the cavity plate 44, so that the cavity plate 44 is curved or deformed in the direction in which the pressure is applied. As a consequence, a little pressure of the ink is propagated to and absorbed by the cavity plate 44, whereby an ejection speed at which the ink is ejected from the ink ejection holes 45a is lowered. Thus, the ink ejection amount is decreases.
In the other type of crosstalk, when all piezoelectric elements 42a are contracted, the pressure in the inside of the ink reservoirs 44b is considerably lowered so that the ink rapidly flows into the inside of the ink reservoirs 44b. Then, the pressure of the ink in the inside of the ink reservoirs 44b becomes excessively large. Because of the characteristics of liquid ink, the pressure state of the ink is balanced. The ink flows backward from the ink reservoirs 44b to the ink ducts 44a. In this manner, the ink alternately flows from the ink reservoirs 44b to the ink ducts 44a or vice versa, thereby causing the pressure wave to occur within the ink. Then, the pressure wave occurs at every ink reservoir 44b and interferes with the ink ducts 44a. Further, the interfering pressure wave interferes with the pressure applied to the ink to lower the pressure of the ink.
Since the pressure produced by expanding the piezoelectric elements 42a is lowered by the crosstalk phenomenon, the ink ejection speed is lowered, and at the same time, the ink ejection amount is decreased. As a result, when the ink is ejected from the actuator 40 while the head scans the print medium, the ejected ink drops are attached to the print medium at its portions different from the expected portions. Moreover, the ink ejection amount changes depending on the number of the piezoelectric elements 42a to be driven. Thus, an image printed on the print medium changes unevenly degrading a sharpness of the printed image.